EARLY DEVELOPMENT OF THE SEA URCHIN
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FIG. 8. Changes in proteins during the embryonic development of Hemicenirotus pulcherrimus demonstrated by the protective enzyme test. Per-cent concentration of proteins specific for: 1. unfertilized eggs; 2. newly fertilized eggs; 3. 8-cell stage; 4. swimming
blastula; 5. early gastrula; 6. late gastrula. (From Ishida and Yasumasu, 1957.)
the primary mesenchyme appear, they degenerate as soon as they reach
the blastocoel (Figs. 9a and b). If the treatment is applied somewhat
later, the cells of the primary mesenchyme have a normal appearance
but the skeleton is defective. Since the effect of ethionine is not counteracted by a successive treatment with methionine it was concluded that
ethionine had been used in the synthesis of some metabolite essential for
the differentiation of the cells of the primary mesenchyme. As a working
hypothesis, it has been suggested that this may be a protein or a group
of proteins. The interesting point here is that this protein, or whatever
metabolite it may be, is synthesized quite early in the course of
development, only to be used, however, considerably later.
These observations therefore indicate: (1) That the period of the first
5 or 6 cleavages, which is the phase during which the rate of oxygen
consumption increases rather slowly, is fairly active in protein synthesis ;